Issue Archive

Bob Reisse coordinates the design and testing of ALHAT (Autonomous Landing Hazard Avoidance Technology) sensors. In December, ALHAT instruments were melded to HUEY helicopters, which used sensors and an integrated computer system to provide guidance and assist pilots. The technology will also enable landing near specific resources and locations across the solar system, including the moon, Mars, and other asteroids.

Bob Reisse: ALHAT is a series of sensors that can determine or measure the area of interest that we’re trying to get to on the ground. In addition to that, we have a standard altimeter just to help us navigate to the right location. The third [part] is a laser Doppler system, which measures attitude and velocity relative to the ground. As you can imagine, an inertial measurement unit (IMU) tells you your velocity, but it doesn’t tell you how you’re doing relative to the ground or to the area of interest as you’re approaching a planetary body.

There’s also a series of computers that take that data and interpret the target location into what we call a digital elevation map, which gives us locations of things that we could consider hazardous. Then that has to be processed into a series of target locations that is then passed on to the guidance computer. The guidance computer then has to look at the trajectory that it is currently on, how it can make an adaption to one of the target locations, and figure out which one is the best one to get to. So sensors, computer systems, and attitude control systems all go into making one complete ALHAT package.

NTB: What will ALHAT be able to do?

Reisse: You have to get to your target location as accurately as you possibly can. In your descent from an orbit, you have to navigate to that location. Once you’re close to the target region, you have to be able to sense what’s at the site so you can determine where it is you want to go, and then you have to navigate to that particular site. We try to do that to the order of meters.

NTB: From a technological perspective, what allows ALHAT to avoid obstacles?

Reisse: The principal instrument that does that is what we call a Flash LIDAR. Instead of being a single detector that determines the distance, the one we’re presently using has 128 x 128 different sensors. So when we send out one flash of light from an instrument, we see 16,000 returns. Those returns can help us determine the structure of the target landing site. From that, you build up a map of the area. We are at present trying to determine essentially where all the hazards are that are about 30 centimeters, or about the size of a basketball, in an area of 100 meters by 100 meters.

NTB: How is that map built out?

Reisse: Currently, we essentially image it as a series of small squares, or small images at about the resolution of the 30 centimeters that we’re talking about, and then build up a sort of semi-raster scan of the entire area. That is then melded together by software.

NTB: From a technology perspective, how will ALHAT enable a safe landing?

Reisse: We’re looking to make sure that the spacecraft lands at a flat enough space that it can do whatever it is that it has to do. In the lunar landings, they had to make sure that they could relaunch the spacecraft to get back to Earth; they needed to be within 11 degrees of the vertical.

One thing we’re looking for is slopes. We don’t want to land on too high of a slope. In the Mars missions, for example, one of the landers had to be fairly close to a few degrees relative to the surface for it to actually work and go about its job. That’s an important consideration, and we want to make sure that we can land in an area that is relatively flat, and doesn’t have any rocks or holes that it would put a foot into. It’s an amalgam of “Where are the craters? Where are the rocks? Where are the holes? And where can we find an area where the spacecraft can put down all its feet safely and be within whatever tolerance the spacecraft has for whatever its future mission is?”

NTB: What other ALHAT technologies assist with navigation?

Reisse: The standard altimeter is used essentially from the time you de-orbit to determine your position. By tracking the terrain as you’re descending to the surface, you can figure out, if you have a decent model of the terrain at the body, where you are relative to the target location.

The laser Doppler system has three beams pointed towards the ground. Each one of them can tell you the relative distance to the ground and the relative velocity along that beam. In terms of navigating to your safe spot, this gives you your information relative to the ground as to your attitude and to your velocity, which are very important in figuring out how to put the spacecraft down into a precise spot. One of the difficulties we basically have is in a lot of missions, if you try to navigate to a safe spot, you really have to center yourself above the spot you want to go and descend vertically. And to do that, we need to make sure that the spacecraft is really not moving.

Question of the Week

This week's Question: In recent years, hundreds of millions of dollars have been invested into space ventures. SpaceX, an advanced spacecraft manufacturer founded by Elon Musk, has completed more than 30 successful launches since 2006, delivered...